I wanted better speakers, have a really good compo from Hifonics (Atlas series) and a limited budget. So I decided to make it fit! Here is how I did it.
I started out with making speaker rings for the woofers as obviously they’re a different size and shape. I chose plywood as the base at the side of the outside and mdf on top of that for easier mounting of the speakers with screws and because if being more massive and therefor less prone to resonating. Also, plywood is better resistant to water. The base of plywood would get the shape of the original speaker and I wanted to mount a circular ring on top of it .
The holes in the plywood are for mounting it to the inner door panel. I made bigger holes in the mdf so I could directly put the screw in the plywood. Then in between the holes I pre-drilled holes for the woofer.
I then started trimming the plywood to make it more compact, only to find out it could have easily been the same shape as the ring. The ring, by the way, is 8" outer diameter and the inner diameter is as specified by my speaker manufacturer. I made the second ring out of two rings, one of plywood and one of mdf. Both the same size except I let in a little edge in the plywood so it’s less likely the speaker falls out during installation. I have also put a little dampening in the door, 1,5 matt probably. Applying the 20/80 rule, get 80% of the result with 20% of the trouble/matts and thus weight.
I then set off creating a solution for the tweeters. I took a good look and decided to make a bracket of mdf with a hole just big enough to sandwich the tweeter in the hole. I can even aim it slightly upwards now 
Forgot to take pictures of the end result mounted but they are painted now and drying. Pics later in this story.
I also needed to find a place for the filter. There wasn’t any place where it fitted straight. There was one place where I could fit the filter with a piece of wood in a specific shape. This was the result:
I painted the parts to withstand moist but also to not be visible. Behind the grills in the door panels you can see the color of the wood. Ugly as hell. Tweeter holders in the right top corner. I’ll make detailed pics with mounted tweeters later.
Then one last challenge. As can be seen in the first picture, the speaker has a baffle to protect it from water and other stuff entering the door at the opening for the windows. There are products shaped for the standard size speakers but they are really expensive for a piece of plastic or foam. They’re easily 30 pounds or more delivered to my door. So I tried to think of something else. I came up with the idea to make a shield of ring binder plastic 
Costed me 1 euro and after a bit of cutting it fits perfectly.
Tommorow 1 more layer of paint, sunday I only need to damped the passenger side door and then I’m ready to install everything and have great sound! YAY!